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Connecting SCSI Cables and Devices
Choosing SCSI Cables
Always use high-quality SCSI cables to connect the host adapter to
devices on the SCSI bus. Poor-quality cables can cause data corrup-
tion, parity errors, and other problems. High-quality cabling is espe-
cially critical if you use UltraSCSI or Fast SCSI data transfer rates.
The round cables used for external SCSI devices vary widely in
quality and electrical characteristics. Be sure external SCSI cables
meet SCSI-2 standards (ask your cable vendor). For UltraSCSI appli-
cations, SCSI cables must meet Fast-20 SCSI standards. Here are
some guidelines for external SCSI cables:
SCSI-2: Use cables with a single-ended impedance range of 90
to 132 ohms.
UltraSCSI: Use cables with a single-ended impedance range of
90 ± 10 ohms. For the
should be 90 ± 6 ohms.
Do not use cables shorter than 30 cm (11.8 inches) between any
two SCSI devices.
Adaptec sells high-quality internal and external UltraSCSI and
SCSI-2 cables. If your reseller does not stock these cables, call
Adaptec directly at the number listed on page ii.
Maximum Cable Lengths
To assure reliable operation, the total length of the SCSI bus, includ-
ing both internal and external cabling, should not exceed 6 meters
(19.7 feet) for synchronous or asynchronous data transfer rates, and
3 meters (9.8 feet) for UltraSCSI data transfer rates.
Caution:
AHA-3940U/3940UW/3940UWD host adapters
support only single-ended SCSI devices. Do not connect
differential SCSI devices, or the host adapter may be dam-
aged. Read your device documentation to find out whether
the device is single-ended or differential.
AHA-3940U/3940UW/3940UWD User's Guide
Stock Number: 511041-00, Rev. B
Print Spec Number: 494569-00
Current Date: 2/14/97
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
Installing the Host Adapter and SCSI Devices
and
signals the impedance
REQ
ACK
Page: 2-5
ECN Date: 2/17/97
2-5
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